Cleaner fish are sensitive to what their partners can and cannot see.

Much of human experience is informed by our ability to attribute mental states to others, a capacity known as theory of mind. While evidence for theory of mind in animals to date has largely been restricted to primates and other large-brained species, the use of ecologically-valid competitive contexts hints that ecological pressures for strategic deception may give rise to components of theory of mind abilities in distantly-related taxonomic groups. In line with this hypothesis, we show that cleaner wrasse (Labroides dimidiatus) exhibit theory of mind capacities akin to those observed in primates in the context of their cooperative cleaning mutualism. These results suggest that ecological pressures for strategic deception can drive human-like cognitive abilities even in very distantly related species.

Western lowland gorillas (Gorilla gorilla gorilla) do not show an aversion to inequity in a token exchange task.

Although individuals in some species refuse foods they normally accept if their partner receives a more preferred one, this is not true across all species. The cooperation hypothesis proposes that this species-level variability evolved because inequity aversion is a mechanism to identify situations in which cooperation is not paying off, and that species regularly observed cooperating should be more likely to be averse to inequity. To rule out other potential explanations of inequity aversion, we need to test the converse as well: species rarely observed cooperating, especially those phylogenetically close to more cooperative species, should be less likely to be inequity averse. To this end, we tested eight zoo-housed Western lowland gorillas (Gorilla gorilla gorilla) on a token exchange task in which subjects received either the same food reward or a less-preferred reward for the same or more effort than their partner, recording both refusals to participate in the exchange and refusals to accept the reward. Supporting the cooperation hypothesis, even with procedural differences across sessions, gorillas were significantly more likely to refuse in all conditions in which they received a low-value food reward after completing an exchange, regardless of what their partner received, suggesting that gorillas were not inequity averse, but instead would not work for a low-value reward. Additionally, gorillas were more likely to refuse later in the session; while the pattern of refusals remained unchanged after accounting for this, this suggests that species should be tested on as many trials as is practical.

Psychopaths fail to automatically take the perspective of others.

Psychopathic individuals display a chronic and flagrant disregard for the welfare of others through their callous and manipulative behavior. Historically, this behavior is thought to result from deficits in social-affective processing. However, we show that at least some psychopathic behaviors may be rooted in a cognitive deficit, specifically an inability to automatically take another person's perspective. Unlike prior studies that rely solely on controlled theory of mind (ToM) tasks, we employ a task that taps into automatic ToM processing. Controlled ToM processes are engaged when an individual intentionally considers the perspective of another person, whereas automatic ToM processes are engaged when an individual unintentionally represents the perspective of another person. In a sample of incarcerated offenders, we find that psychopathic individuals are equally likely to show response interference under conditions of controlled ToM, but lack a common signature of automatic ToM known as altercentric interference. We also demonstrate that the magnitude of this dysfunction in altercentric interference is correlated with real-world callous behaviors (i.e., number of assault charges). These findings suggest that psychopathic individuals have a diminished propensity to automatically think from another's perspective, which may be the cognitive root of their deficits in social functioning and moral behavior.

What do monkeys know about others' knowledge?

Recently, comparative psychologists have suggested that primates represent others' knowledge states. Evidence for this claim comes from studies demonstrating that primates expect others to maintain representations of objects when those objects are not currently visible. However, little work has explored whether nonhuman primates expect others to share the more sophisticated kinds of object knowledge that they themselves possess. We therefore investigated whether primates attribute to others knowledge that is acquired through the mental transformation of a static object representation. Specifically, we tested whether rhesus macaques (Macaca mulatta) expected a human demonstrator to solve a difficult rotational displacement task. In Experiment 1, monkeys watched a demonstrator hide a piece of fruit in one of two boxes. The monkey and the demonstrator then watched the boxes rotate 180°. We found that monkeys looked longer when the demonstrator reached into the box that did not contain the fruit, indicating that they expected her to be able to track the fruit to its current location. In Experiment 2, we ruled out the possibility that monkeys simply expected the demonstrator to search for the food in its true location. When the demonstrator did not witness the rotation event, monkeys looked equally long at the two reaching outcomes. These results are consistent with the interpretation that rhesus macaques expect others to dynamically update their representations of unseen objects.

A decade of theory of mind research on Cayo Santiago: Insights into rhesus macaque social cognition.

Over the past several decades, researchers have become increasingly interested in understanding how primates understand the behavior of others. One open question concerns whether nonhuman primates think about others' behavior in psychological terms, that is, whether they have a theory of mind. Over the last ten years, experiments conducted on the free-ranging rhesus monkeys (Macaca mulatta) living on Cayo Santiago have provided important insights into this question. In this review, we highlight what we think are some of the most exciting results of this body of work. Specifically we describe experiments suggesting that rhesus monkeys may understand some psychological states, such as what others see, hear, and know, but that they fail to demonstrate an understanding of others' beliefs. Thus, while some aspects of theory of mind may be shared between humans and other primates, others capacities are likely to be uniquely human. We also discuss some of the broader debates surrounding comparative theory of mind research, as well as what we think may be productive lines for future research with the rhesus macaques of Cayo Santiago.

Capuchins (Cebus apella) are limited in their ability to infer others' goals based on context.

Recent research suggests that many primate species understand others' actions not only in terms of their physical movements, but also in terms of the actor's underlying goals and intentions. Impressively, apes also have the capacity to incorporate previously acquired contextual information into their goal representations. To date, little work has tested whether other primates demonstrate this level of flexibility when inferring others' goals. To help address this question, we tested capuchin monkeys using a procedure similar to one that Buttelmann, Schütte, Carpenter, Call, and Tomasello (2012) used to test apes. Capuchin subjects were repeatedly shown an experimenter manipulating locking mechanisms on a series of boxes. In an experimental condition, the experimenter gave subjects food retrieved from inside the boxes, whereas in a control condition subjects never received food from inside the boxes. We then explored how capuchin subjects would interpret the experimenter's ambiguous manipulation of a novel box. In contrast to apes, subjects in our experiment showed little evidence of being able to flexibly use temporally dissociated contextual cues to make inferences regarding others' goals. This result may point to a crucial difference in the sophistication with which ape and nonape primates understand others' actions.

Insights into Intraspecies Variation in Primate Prosocial Behavior: Capuchins (Cebus apella) Fail to Show Prosociality on a Touchscreen Task.

Over the past decade, many researchers have used food donation tasks to test whether nonhuman primates show human-like patterns of prosocial behavior in experimental settings. Although these tasks are elegant in their simplicity, performance within and across species is difficult to explain under a unified theoretical framework. Here, we attempt to better understand variation in prosociality by examining the circumstances that promote and hinder the expression of prosocial preferences. To this end, we tested whether capuchin monkeys (Cebus apella)-a species that has previously demonstrated prosocial preferences-would behave prosocially using a novel touchscreen task. In contrast to previous studies, we found that capuchins as a group did not prosocially deliver food to a partner. Importantly however, data from control conditions revealed that subjects demonstrated limited understanding of the reward contingencies of the task. We also compared individuals' performance in the current study with their performance in a previously published prosociality study. We conclude by discussing how continuing to explore intraspecies variation in performance on prosocial tasks may help inform debates regarding the existence of other-regarding preferences in nonhuman species.

Capuchins' (Cebus apella) sensitivity to others' goal-directed actions in a helping context.

As humans, our ability to help others effectively is at least in part dependent upon our capacity to infer others' goals in a variety of different contexts. Several species of nonhuman primate have demonstrated that they will also help others in some relatively simple situations, but it is not always clear whether this helping is based on an understanding of another agent's goals. Although the results of a number of different studies support the hypothesis that chimpanzees represent others' goals in various helping contexts and are sensitive to these goals when actually helping others, less work has addressed whether more distantly related species actively represent goals when helping. To explore the cognitive mechanisms underlying helping behaviors in species less closely related to humans, we tested whether a species of New World monkey-the brown capuchin (Cebus apella)-would provide an experimenter with a desired out-of-reach object more often than an alternative object when the experimenter attempted to obtain the former object only. We found that capuchins reliably helped by providing the experimenter's goal object (Experiment 1) and that explanations based on the use of several less sophisticated strategies did not account for the overall pattern of data (Experiments 2-4). Results are thus consistent with the hypothesis that capuchins help others based on an understanding of their goals although more work is needed to address the possibility that capuchins may be responding to gestural and postural factors alone.

Endowment effects in gorillas (Gorilla gorilla).

Reports of endowment effects in nonhuman primates have received considerable attention in the comparative literature in recent years. However, little is known about the mechanisms underlying these effects. Continuing to explore endowment effects across different species of primate may reveal subtle differences in behavior that can help formulate specific hypotheses about the relevant mechanisms and the social and ecological factors that have shaped them. In this study, we use a paradigm that has previously been used to test chimpanzees (Pan troglodytes) and orangutans (Pongo spp.) to explore whether western lowland gorillas (Gorilla gorilla) exhibit comparable endowment effects. We find that gorillas exhibit endowment effects when in possession of food, but not nonfood, items, and that they show a statistically stronger effect than chimpanzees but not orangutans. These findings are consistent with the hypothesis that mechanisms for endowment effects in primates may be related to inhibitory control or risk aversion.

Visitor effects on the behavior of captive Western lowland gorillas: the importance of individual differences in examining welfare.

Given the conflicting and somewhat limited findings available on the effect of zoo visitors on primate behavior, the primary purpose of this study was to provide additional data on gorillas' response to variations in crowd size and to look at what other factors-both intrinsic (e.g. personality, sex, and rearing history) and extrinsic (e.g. group)-might influence such responses. Subjects included four groups (three mixed-sex and one bachelor) of captive western lowland gorillas living at Zoo Atlanta. Overall activity budgets and rates of social behaviors in high and low crowd conditions were compared. Behaviors suggestive of general welfare did not vary as a function of crowd size alone, although we did observe one significant interaction and several trends. Specifically, two groups spent more time engaged in undesirable behaviors in the high crowd condition, whereas the other two groups showed the opposite response pattern. Additionally, males, but not females, showed a trend toward greater rates of contact aggression in the high crowd condition, although wounding rates did not vary with crowd size. We also found trends towards variation in response to crowd size as a function of individual personality ratings. These results suggest that although the visitor effect may constitute an impediment to optimal animal welfare, this may only apply to some individuals or groups. We emphasize the importance of continuing to explore individual differences and the limitations of a one-size-fits-all approach when describing influences on animal welfare.